1. Field of the Invention
The present invention relates to a multilayer interconnection circuit board having an interlayer wiring arrangement formed by irradiating laser beams.
2. Discussion of Background
Recently, a demand of miniaturizing electronic devices and appliances has been increased, and a demand of forming conductors having high density has been required for multilayer interconnection circuit boards. As means for forming highly densed conductors, there has been studied a technique of connecting electrically conductor wiring layers between which insulating layers are interposed.
FIGS. 3a and 3b are respectively cross-sectional views showing a conventional method of connecting layers in a multilayer interconnection circuit board disclosed in, for example, Japanese Unexamined Patent Publication No. 84594/1984. FIG. 4 is a perspective view in order to make FIG. 3a easily understand. In FIGS. 3 and 4, a reference numeral 1 designates an insulating layer made of a polyimide material, a numeral 2 designates a conductor wiring layer made of a material such as copper, a numeral 3 designates a Nd:YAG laser beams, a numeral 4 designates a window formed in the conductor wiring layer 2 in order to irradiate the laser beams, and a numeral 5 designates a char region formed in the insulating layer 1 of polyimide by irradiating the YAG laser beams 3.
When the YAG laser 3 is irradiated through the window 4, a part of the insulating layer 1 of polyimide is charred, whereby two conductor wiring layers interposing the insulating layer 1 therebetween are electrically connected through the char region 5.
FIGS. 3a and 4 shows an example wherein the window 4 is formed in a conductor wiring layer 1. However, an example wherein the conductor wiring layer 1 has no window 4 can be considered. In this case, the electrical connection of two conductor wiring layers is performed as follows. An laser output is increased so that the upper conductor wiring layer 1 is molten. Then, the laser output is returned to the original level, and a char region 5 is formed in the same manner as that as shown in FIG. 3b.
In the conventional technique of electrically connecting the multilayer interconnection circuit board described above, it was necessary to adjust a laser output depending on the presence or absence of the window 4. There was, therefore, a problem that a high speed machining operation could not be obtained.